We intend to determine the structure, at atomic resolution, of the rabbit cationic protein NP2 and other related cationic proteins. An excellent crystal form of NP2, which diffracts beyond 1.5 angstrom resolution, has been grown. NP2 is one of a class of small peptides,the cationic proteins, which are found in neutrophils and machrophages. Cationic proteins are highly effective antimicrobial agents in vitro, with a very wide spectrum of activity: they destroy fungi and Gram-negative or Gram-positive bacteria, and they prevent viral replication in fibroblasts, all at concentrations (1-20 ug/mL) commensurate with those of traditional antibiotics like penicillin. NP2 contains 33 amino acid residues, six of which are cysteine. The space group of our crystal form is P21, the same as that of the lipophilic peptide crambin which contains 46 residues with six cysteines. The crambin structure was determined by first solving the positions of its sulfur atoms and then using phases predicted from sulfurs only, as a partial structure, to interpret the structure of the rest of the protein. The sulfur locations were determined through the use of accurately measured anomalous dispersion of the sulfur atoms. We intend to use this method to solve the structure of NP2. The cationic proteins offer an attractive promise as pharmacologic agents. If we can understand the mechanisms of their activity, we probably will discover important principles of antimicrobial therapy. The knowledge of their structures will be vital to that understanding. The six known rabbit cationic proteins vary substantially in antimicrobial activity yet are highly homologous in peptide sequence. If their variations in tertiary structure can be correlated to variations in their activities, we may learn to artificially design more effective antibiotics in a rational way.